Electric automobiles and hybrid cars use an electric motor as a power source or an auxiliary power source. A secondary battery configured of plural single cells connected to one another in series is used as a power source for this type of electric motor. For example, Japanese Patent Application Laid-open Publication No. 2001-204141 has described a secondary battery configured of 60 nickel-metal hydride batteries with a standard voltage of 1.2 V per single cell which are connected to one another in series.
In the case of such nickel-metal hydride batteries, the battery voltage of each single cell is low, and no more than approximately 70 V is obtainable even from the 60 cells connected to one another in series. This makes nickel-metal hydride batteries unsuitable for electric automobiles and the like which are driven with a high voltage and a high output. Another problem with nickel-metal hydride batteries is that the nickel-metal hydride batteries self-discharge when fully charged.
By contrast, lithium-ion batteries are advantageous, for example, in that a high voltage of 300 V to 1000 V and a high output are obtainable, and in that no lithium-ion batteries self-discharge even when fully charged. Against this background, there is an idea about use of a battery pack in which single-cell lithium-ion batteries are connected to one another in series.
Furthermore, in a case of a voltage detecting device of a battery pack for an electric automobile described in Japanese Patent Application Laid-open Publication No. Hei. 11-160367, for example, a main battery for running power storage with a high voltage of 300 V is divided into plural battery modules; thus, the module voltages of the respective battery modules are detected by a differential voltage detection circuit and an A/D converter circuit; and the detected module voltages are transmitted to a signal processing circuit unit. Power is supplied to the differential voltage detection circuit and the A/D converter circuit from an auxiliary battery, but not from the main battery, via a DC-DC converter. This makes it possible to detect the open-circuit module voltages with high precision, and to accordingly avoid unnecessarily shortening the life of the battery pack with the high voltage.
Moreover, a power storage device described in Japanese Patent Application Laid-open Publication No. 2003-70179 includes: plural power storage modules connected to one another in series, each power storage module having plural capacitors which are connected to one another in series; plural subordinate control units, provided correspondingly to the plural power storage modules, and each for controlling the plural capacitors constituting the corresponding power storage modules; and a host control unit for controlling the plural subordinate control units. Each subordinate control unit detects the conditions respectively of the plural capacitors which constitutes the power storage module controlled by the subordinate control unit, and obtains a logical disjunction (OR) or a logical product (AND) of a resultant condition detection signal with an input signal from the high-potential subordinate control unit, thus outputting the result to the low-potential subordinate control unit. This content is described in claims 4 and 5 in the above-mentioned patent official gazette.
In addition, a battery protection IC described in Japanese Patent Application Laid-open Publication No. 2005-117780 is to monitor a block consisting of plural batteries connected to one another in series. The battery protection IC includes: an overvoltage detection circuit for outputting an overvoltage signal in a case where a voltage at a terminal in any one of the batteries in the block becomes equal to, or more than, a prescribed value while the battery pack is being charged; an overdischarge detection circuit for outputting an overdischarge signal in a case where a voltage at a terminal of any one of the batteries becomes equal to, or less than, a prescribed value while the battery pack is being discharged; a first switch for, when detecting the overvoltage signal, turning on and informing that the overvoltage has occurred; and a second switch for, when detecting the overdischarge signal, turning on and informing that the overdischarge has occurred. This content is described in Paragraphs [0024] and [0025] in the above-mentioned patent official gazette.